1. Technical Field
The present invention relates to production process for a Polyester Resin. In particular, this invention relates processes for manufacturing polyester resins. Still particularly, this invention relates to a catalyst system for manufacturing polyester resins.
2. Background and Prior Art
Carothers and Hill in the late 1920s were the first to form fiber forming polyester resins using the melt condensation of dicarboxylic acids and aliphatic diols. Poly(ethylene terephthalate) [PET] was invented in the 1940s by Winfield and Dickson and suggested its use for fiber and film formation.
This invention particularly relates to PET resins but is not restricted to PET resins and extends to related Polyester resins such as polyethylene naphthalate (PEN), polybutylene naphthalate (PBN) and polytrimethylene naphthalate (PTN) and process of preparing these resins. Although, the description and examples will be described with reference to PET resin, it will be appreciated by one skilled in the art that the results and processes can be extended to other polyester resins.
PET is produced by two routes: the dimethyl terephthalate route, [DMT] route, which involves the use of ethylene glycol and dimethyl terephthalate. The process requires a high degree of purity of the monomer. With the availability of sufficient quantities of purified terephthalic, [PTA], increasingly, the 2nd route, PTA route, has now become more popular.                a. The PTA route of making the polyester resin such as PET follows a series of steps involving intermolecular reactions between bi-functional and poly-functional compounds: In the first step a paste is made of the acid by preparing in a vessel, a paste comprising:                    (i) diacid component, typically a solid, selected from a group consisting of Terephthalic Acid (TPA) and Naphthalene Dicarboxylic Acid (NDA) or a diester component consisting of dimethyl ester of the acid viz. Dimethyl Naphthalene Dicarboxylate (NDC) and            (ii) diol component, typically a liquid, selected from a group consisting of mono ethylene glycol (MEG), Trimethylene Glycol, 1,3-Propylene Diol (PDO) and Tetramethylene Glycol, Butylene Glycol and 1,4-Butane Diol (BDO).                        
In the case of PTA and ethylene glycol [EG] the paste is charged in a typically a first esterification stirred tank reactor maintained at around 230 to 280 degrees Celsius and pressure in the range of 0.5 to 3.0 kg/cm2 and for a period ranging between 3 and 4 hours, resulting in an esterification process leading to the formation of a prepolymer, consisting of bis-hydroxyethyl terephthalate [BHET] and short chain oligomers. The esterification is not complete and some acid end groups remain in the prepolymer. Water, an esterification by-product, is removed by a column system and [EG] is recycled.
The first esterification step resulting in the prepolymer is typically followed by a polycondensation step in which a trans-esterification and an esterification reaction leads to step-growth polymerization in the melt phase. These reactions are reversible and require that the condensates [EG] and water are efficiently removed from the melt, typically by using reduced pressure. Initially, when the melt viscosity remains relatively low, the condensate products can be removed easily by evaporation. However, as the viscosity increases bubble formation is hindered and diffusion of EG is required. This may be also achieved by using a disk ring reactor to reduce the diffusion path, creating a thin renewable film thereby increasing the surface area and removing the condensates. A three stage reaction may also be utilized in which the esterification is carried out at a temperature in the range of about 230 to 260° C. and pressure in the range of 0.1 to 0.5 kg/cm2 and for a period ranging between 2 to 2.5 hours while maintaining inert atmosphere using nitrogen circulation, followed by pre polymerization carried out at reduced pressure in the range of 5 and 15 mbar and at a temperature in the range of 260 and 285° C. for a period of about 30 minutes, and ending by polymerization carried out at a reduced pressure in the range of 0.1 to 0.5 mbar at a temperature in the range of 280 and 298° C., preferably between 280 and 285° C. for a period ranging between 100 and 155 minutes.
At the end of the reaction the melt can be used directly for making fibers. However, in melt polymerization it is difficult to obtain PET of number average molar mass Mn greater than 20,000 g/mole [intrinsic viscosity, IV ˜0.6 dL/g] needed for making bottles and industrial fibers, because of reduced mass transfer rates and chemical degradation at high temperatures and long residence time. However in the disc ring reactor it is possible to reach I.V. of ˜0.8 without degradation, and with more antimony catalyst even higher. Therefore, it is usual to follow melt polymerization by solid state polymerization [SSP]. The melt is extruded into strands which solidify on cooling and the strands are cut into essentially amorphous chips for the post polymerization process. In SSP the amorphous chips are first subjected to crystallization at 100 to 170 degrees C. to avoid sticking during the subsequent processing at higher temperatures. The crystalline material is then charged into the SSP reactor at 200 to 230 degrees C., typically 50 degrees C. lower than the melt phase reaction. The reactions in SSP are essentially the same as in the melt phase except that the reaction takes place in the solid phase. EG is removed either by nitrogen sweeping or by application of reduced pressure or vacuum. Reaction temperature drop reduces the polycondensation rate by a factor of 6, while the chemical degradation rate drops by a factor of 40. Moreover, problems associated with stirring of the viscous melt are eliminated and much more important is the reduction of acetaldehyde. PET for instance, of average molar mass Mn upto 27,000 g/mole [intrinsic viscosity, IV ˜0.8 dL/g] [for bottles] and as high as 38,000 g/mole [intrinsic viscosity, IV ˜1.20 dL/g] [for industrial fibers] can be achieved.
It is now well known that use of one or more catalysts during ester interchange and polycondensation both in melt polymerization and in solid state polymerization speeds up the rate of polymerization. Sequestering agents, typically phosphorus compounds, are required particularly in transesterification reactions to control catalyst activity to prevent thermal degradation and discoloration of the resultant polymer. U.S. Pat. Nos. 2,641,592, 3,028,366, 3,732,182, 3,795,639 3,842,043, 3,907,754, 3,962,189, 3,965,071, 4,010,145, 4,082,724, 5,008,230, 5,019,640, 5,116,938, 5,162,488 and 5,166,311 and JP 2000086751 all deal with processes for preparing linear polyesters utilizing variety of catalysts comprising Germanium, Antimony, Titanium, Manganese, Cobalt, Zinc, Magnesium, Calcium, Aluminum, Bismuth, Iron, Copper etc. and variety of phosphorous containing compounds as heat stabilizers.
Various additives are reported in the literature for increasing the SSP rate. U.S. Pat. Nos. 6,699,545 and 5,644,019 teach use of zinc p-toluenesulfonate as SSP rate accelerators.
Polyfunctional compound additives like trimellitic anhydride (TMA), pyromellitic dianhydride (PMDA) etc. are known as SSP accelerators as described in U.S. Pat. Nos. 5,338,838, 5,334,669 and 5,243,020 and EU 422282.
U.S. Pat. No. 5,382,650 describes a process wherein Tungsten Sulfide (WS2) in combination with intercalated alkali metal cations like Lithium is used as an esterification and/or transesterification catalyst and the product is for molding applications.
Japanese Patent Application 56020028 (1981) uses ethylene glycol soluble tungsten compounds like Tungstic Acid, Tungsten Chloride, Tungsten pentaphenoxide etc along with polycondensation catalysts like Antimony Trioxide, Germanium dioxide, Zinc acetate etc. and claims an improvement in color and a reduction in melt polymerization time. A significant limitation of the process disclosed in this application is that the L value with Tungstic Acid as the additive along with Antimony Trioxide shows a drop in the value from 75.1 (without the Tungstic Acid additive) to 74.3, though it is not significant.
In our invention the tungsten compound used, though insoluble, has shown considerable decrease not only in melt polymerization time but also a significant improvement in L value. Additionally we have seen an increase in the SSP rate about which there is no mention in the Japanese patent. One of the reasons for the insoluble tungsten compound which has been very effective in our invention can be due to the fine particle size (˜4 micron) which gets thoroughly dispersed in the polymer melt and manifests its catalytic effect both in melt poly and SSP.
Again, in JP 5674123 (1981)—the inventors have used glycol soluble Tungstic Acid, Tungsten Chloride and Tungsten Pentaphenoxide. To improve the color they have used additionally phosphorous compounds. They have been able to achieve an accelerated melt polymerization rate and disclose consistently high L values—all in the range of 80 to 82.
When the experiments were repeated by the herein inventors with Tungstic Acid it showed acceleration in melt poly but there was no increase the SSP rate.
Again, in JP 61293220 (1986) a process of polymerization with tungstic Acid is disclosed wherein a heating medium, either paraffin, or one or more compounds of biphenyl derivatives, is added along with the raw materials containing Tungsten compound and the regular catalysts and the agitation is by blowing inert gas like nitrogen. According to this disclosure one need not do SSP and with this process they are able to reach a minimum viscosity of 2.3. There is no mention of rate of polymerization and color of polymer. This work is more academic in nature as industrially this process is not practical because contact of the heating medium directly with the polymer melt will give rise to contaminant problems.
JP 2117950 (1990) discloses a process for PET preparation for film application with Tungsten Trioxide grains with an average size of 0.01 to 5 micron. They claim excellent transparency in the film. This process deals with Melt polymerization but there is no mention of rate, or L Values. Also this application anticipates the important feature of particle size as mentioned earlier in our invention.
U.S. patent applications 20050153086 dated Jul. 14, 2005, 20040236066 dated Nov. 25, 2004 and 20050261462 dated Nov. 24, 2005 deal with slow crystallizing resins, resins for application such as hot fill beverage containers and titanium catalyzed polyester resins respectively; and they have claimed the use of Tungsten compounds as one of the several heat up rate additives loaded in a carrier resin as master batch for achieving improved reheating profile in performs through blending of this master batch; the dosage of the tungsten compounds or other additives are not recommended for addition in melt phase reactor but only as a master batch in injection molding machine; the resin is also a copolymer with added comonomers. There is no suggestion in this patent application for the use of the catalyst system in the solid state polymerization step of the reaction.
The underlying theory on which this invention is based is the fact that compared to melt polymerization, there is significantly less degradation of the polymer in solid state polymerization and higher intrinsic viscosity build up. If the solid state polymerization rate is accelerated, then apart from the fact that there will be greater productivity, there will also be lesser degraded product as a result of decreased time of thermal exposure. This lesser degradation results in lower generation of acetaldehyde.
None of the above mentioned prior art references, provides an additive for specifically improving the solid state polymerization rate and concomitantly having no effect on the L value if not an improved L value.
Another aspect of this invention therefore is the improvement of the clarity and colour values of the final product made from the resin processed in accordance with this invention.
The perception of color differs from one person to another.
To overcome the shortcomings of the perception and color interpretation of human eyes, models such as CIE L*a*b* (CIELAB), CIE XYZ were developed to measure and define colors more accurately.
CIELAB was developed by the International Commission on Illumination, a French organization. (Commission Internationale d'Eclairage, hence the CIE acronym in its name). The CIELAB model was originally developed based on the ‘Tristimulus theory’ of color perception and the human eye's response to RGB (Red, Green and Blue colors) which accurately represent human color perception.
The CIE L*a*b* color space, is a device-independent color space that provides a “universal” reference standard.
The L*a*b* space describes colors according to their position along three axes in a 3-D color space: The L axis represents the lightness or brightness of the image and is a measurement of the white-to-black content of any color. The ‘a’ axis runs from red to green and the ‘b’ axis runs from yellow to blue. Position in the L*a*b* space is defined in the following way:                L value from 0 (all black) to 100 (all white)        Position on the a axis, a range from −a (green) to +a (red)        Position on the b axis, a range from −b (blue) to +b (yellow)        Any color can be described mathematically by defining its location within this 3-D physical space.        
The L*a*b* values of a color are generated by measuring the color with instruments such as a Colorimeter, Spectrophotometer, Microspectrometer, Reflectance Tintometers and the like.
The CIELAB color model was created to serve as a device independent, absolute model to be used as a reference.
CIELAB can effectively be employed in industrial color acceptability applications The plastic bottle industry has many producers, each striving to manufacture high quality products. Properties such as barrier characteristics, dimensional stability, shape and appearance are of significant importance.
Appearance attributes can include reflected color, transmitted color, yellowness, and haze. For some applications the presence of proper levels of UV inhibitors is also important. The use of color measurement instrumentation to test and quantify these attributes ensures customer satisfaction and profits.
There are two basic types of color measurement instruments used to measure colors in plastics namely the tristimulus calorimeters and calorimetric spectrophotometers. Color is measured in terms of the tristimulus color scale [the CIE L*a*b* scale discussed above]. Comparisons between a standard and a sample are expressed in terms of delta L*,delta a* and delta b* values.
Using this information, one can interpret both the size of the difference (large or small numbers) and direction of color difference (+ or −).
When determining acceptability, these numbers must fall within predetermined tolerance limits.